In papermaking and paper finishing, a sizing agent is often employed to provide desirable characteristics sought in the ultimate paper product.
Sizing, or sizing property, is a measure of the resistance of a manufactured paper or paperboard product to the penetration or wetting by an aqueous liquid. Sizing agents are internal additives employed during papermaking or external additives employed as coating agents during paper finishing that increase this resistance.
Papermaking can be carried out under acidic, neutral, or alkaline pH conditions, and the selection of a sizing agent is usually dependent on the pH used. For example, rosin-derived sizing agents are typically used under acidic papermaking conditions. Under alkaline pH conditions, which are widely used in fine paper manufacturing applications, typical sizing agents include alkyl ketene or alkenyl dimers or acid anhydrides such as alkenyl succinic anhydrides.
The rate at which the sizing property develops in the sized paper is very important. The sizing property is advantageously developed as quickly as possible after the sizing agent has been added or applied. It is known that the level of size development increases as sized paper is dried to remove moisture. A fast rate of size development is desired for reducing or controlling the water and additive pick-up at the size press of a paper machine. A fast rate of sizing is also important for accurately measuring final paper properties at the end of the paper machine without waiting or additional heating. In papermaking processes where the sizing agent if added at the wet end of the paper machine, the sized paper is typically dried to about 0.8-3 wt. % moisture to obtain adequate development of the sizing property before the paper reaches the size press; at the end of the size press treatment, the paper is typically dried to about 4-6 wt % moisture.                If the sizing property is not fully developed at the end of the paper machine, corrective measures must be taken, e.g., the paper must be stored for sufficient time (hours or days) until the sizing property develops adequately for the intended use of the paper, or an excess of sizing agent must be used to provide adequate sizing property if the benefit is required (e.g., during the paper finishing or converting steps) before the sizing property has completely developed.        
The sizing properties provided by conventional paper sizing agents may be improved by the use of sizing promoters, also called sizing accelerators. Numerous paper sizing promoters are know; see, e.g., U.S. Pat. No. 4,040,984; U.S. Pat. No. 4,764,365; U.S. Pat. No. 4,772,462, U.S. Pat. No. 4,478,682; U.S. Pat. No. 4,847,315; U.S. Pat. No. 4,895,621; U.S. Pat. No. 5,498,648 and U.S. Pat. No. 5,853,542.
Despite the beneficial sizing properties provided by these prior art paper sizing promoters, there is still great demand for further improvement. Promoter resins described in the above patents are detrimental to the effectiveness of optical brighteners that are added to the paper making process to improve the whiteness or brightness of the paper. Therefore, a disadvantage in using conventional sizing promoters is that sizing promoters reduce the effectiveness of optical brighteners which are used to brighten white paper. That is, paper manufactured with sizing agents and sizing promoters will not appear as bright compared to unsized paper each with optical brighteners added. Thus, some of the manufacturing advantage of size promoters is offset by less-bright paper.
Alternately, the interaction of the optical brighteners may inhibit the performance of the sizing promoter. Thus, in order to achieve both paper brightness and promotion of sizing more of these agents must be added to the paper machine.
Cationic polymers and copolymers based on the cyclopolymerization of dimethyldiallylammounium chloride are well know for use in a wide variety of industrial applications. Poly(diallyldimethylammounium chloride) homopolymers are well known cationic polymeric compounds that have been used commercially in papermaking for a wide variety of purposes, e.g., for aiding furnish retention and additive retention in paper; for increasing the dewatering rate of wet paper web; for neutralizing anionic materials in white after; and for size enhancement, to improve paper sizing efficiency and its rate of development. Reten® 203 retention aid (Hercules Incorporated, Wilmington, Del.), a product which contains a diallyldimethylammonium chloride homopolymer, is one such product.
Copolymers and terpolymers containing diallylamine-type compounds, such as diallyldimethylammonium chloride (DADMAC), methylalkiallyl ammonium chloride or diallylammonium chloride (also referred to as DAA.HCl or DAAC), as one of the monomeric components are known. Japanese Patent 57 161197, discloses use of copolymers of sulfur dioxide and diallyldialkylammonium salts, such as DADMAC, or diallylammonium salts, as a dispersing agent with a paper sizing agent. European Patent 282 081, discloses (meth)acrylamide terpolymers that also contain DADMAC or diallylamine, useful in combination with aluminum sulfate for increasing paper strength. Japanese Patent 52 47883, discloses copolymers of acrylamide and diallylamine-type compounds, useful for producing stronger paper. U.S. Pat. Nos. 4,279,794 and 4,295,931, disclose the use of poly(diallylamine) epihalohydrin resins as paper sizing accelerators. Japanese Patent 62 99494 discloses use of copolymers of diallylammonium salts and certain non-ionic water-soluble monomers (e.g. acrylamide) with a paper sizing agent to provide improved sizing property development.
Another approach to improving sizing is reported in U.S. Pat. No. 5,853,542, here the copolymer of DADMAC and DAA.HCl are reported to enhance paper sizing. Since the resultant copolymers are cationic they would be expected to inhibit the effectiveness of optical brighteners. The adverse interaction of cationic paper additives is well documented by William F. Scott in Principles of Wet End Chemistry, TAPPI Press, 1996, page 48.
Other reports in the literature describe polymers of DADMAC and DAA.HCL and optional less than 20% alpha beta carboxylic acids. Japanese Patent Application No. Hei (9) 1997-3793 describes a polymer of diallylamine, methacrylamide and a critical cross-linking monomer and optionally less than 20% of an anionic unsaturated carboxylic acid-based compound polymer system which increases paper strength, improves freeness (measure of pulp drainability), without disturbing the formation of the paper. Their most essential monomers are acrylamide and substituted acrylamides which are not cationic relative to their acrylamide functionality.
Japanese Patent No. Hei (8) 1996-49193 describes polymers derived from hydrophilic vinyl monomers with amino groups and/or quaternary ammonium groups and hydrophobic vinyl monomers. The hydrophilic vinyl monomers would function as cations if they are quanterized. Polymers with up to 5% acrylic acid are shown in this report. The resultant polymers are used to coat papers to provide superior printability.
White paper is achieved by adding optical brighteners in the form of fluorescent dyes. These dyes are very effective when used with highly bleached pulps. These fluorescent dyes absorb light in the ultraviolet region (below 370 nm) and re-emit the light in true visible range (usually the blue region). This gives a fluorescent effect that produces a bright white in daylight masking the inherent yellowness of the bleached pulp. (Principles of Wet End Chemistry, William F. Scott, TAPPI Press, 1996, page 47).
In Principles of End Chemistry, William F. Scott Tappi Press, 1996, page 48, Reynolds describes that it is critical not to add anionic cyes close to the addition point of a cationic additive.
The fluorescent dyes are generally anionic and under use conditions their effectiveness is significantly inhibited by the cationic sizing promoters. Optical brightener producers such as the Clariant Corporation, Charlotte, N.C. or The Bayer Corporation, Pittsburgh, Pa. advise that the optical brighteners should be added at points in the paper making process significantly removed from cationic chemicals such as the common sizing promoter resins.
An attempt at mitigating the adverse effect of sizing promoters on the optical brighteners is described in U.S. Pat. No. 5,498,648. This is achieved by paper size mixtures which are prepared by mixing an aqueous suspension of a digested cationic starch with a finely divided aqueous polymer dispersion and emulsifying a C14-C20 alkyldiketene is this mixture at not less than 70° C. The patent describes that the digested cationic starch and dispersion combine to reduce the negative impact on the paper whiteness.
Despite the reported usefulness of diallyl-based cationic polymers for a variety of industrial purposes, there has not been found any suggestion in the prior art of the usefulness of co- and terpolymers of diallyldialkylammonium salts, optionally, diallylamonium salts, and unsaturated organic acids for improving the sizing property characteristics of sized paper, while not adversely effectiveness paper brightness from optical brightners.